If you must be on that 17:27 train, I think you will have to leaveat about 16:30 with possibly 20 minutes of the match still to go. You will have the added difficulty that you would be leaving before teh stadium shuttles start running back to the city centre and therefore have to get yourself up to the main road to get a bus.
If you were to leave on the final whistle, you would be at the wrong end of the ground for the shuttle buses and would therefore find loads of Cov fans in the queue in front of you for the all-important first buses. And don't forget that even when you get back to the bus station you still have a walk that Fludie reckons takes 10 minutes and I reckon will take almost twice as long. Added to that, if you are catching a train for the north, it must go via Brum and therefore will not leave from the nearest platform so even that adds a minute or two.
Sorry this is not good news. Why don't you ask the same question on one of the Coventry boards? They might be able to come up with something more accurate than my estimations.
And another hint - when you have a particular question to ask (like this one is), then ask that particular question rather than asking something more general - you may then get an answer which is more appropriate to your needs.